01 - Preheat oven to 350°F. Line a baking sheet with parchment paper or prepare a 9 x 13 inch baking pan.
02 - In a large mixing bowl, cream together softened butter and granulated sugar until light and fluffy. Incorporate egg and vanilla extract until blended.
03 - In a separate bowl, whisk all-purpose flour, baking powder, and salt. Gradually mix dry blend into wet mixture until combined.
04 - Press dough evenly into prepared pan, forming a 1/4 inch thick base.
05 - Bake for 13 to 15 minutes or until cookie edges turn golden. Cool entirely on a rack before frosting.
06 - Using an electric mixer, beat cream cheese and softened butter until smooth. Add powdered sugar and vanilla extract, mixing until creamy consistency is achieved.
07 - Spread cream cheese frosting evenly over the cooled cookie base.
08 - Arrange blueberries in the upper left corner to resemble flag stars. Layer sliced strawberries and raspberries in rows to form stripes, alternating with the white frosting.
09 - Refrigerate assembled dessert for at least 30 minutes before slicing and serving.